Patients receive oral benzoylphenylurea weekly for 6 weeks. Courses repeat every 8 weeks in the absence of disease progression or unacceptable toxicity. Cohorts of 3-6 patients receive escalating doses of benzoylphenylurea until the maximum tolerated dose (MTD) is determined. The MTD is defined as the dose preceding that at which 2 of 3 or 6 patients experience dose-limiting toxicity.